Question

Sorry for the naff title, but I’m not really sure how to explain this, I’m one of the new generation whose SQL skills have degraded thanks to the active record patterns!

Basically I have three tables in PostgreSQL

Client (One Client has many maps)
– id

Maps (Map has one client and many layers)
– id
– client_id

Layer (Layer has one map)
– id
– map_id

I would like to write an SQL query that returns Cliend.id along with a count of how many maps that client has and the total number of layers the client has across all maps.

Is this possible with a single query? Speed isn’t of concern as this is just for analytical purposes so will be run infrequently.

Answer 1

I’d use a pair of subqueries for this. Something like:

SELECT
    id,
    (
         SELECT count(map.id)
         FROM map 
         WHERE map.client_id = client.id
    ) AS n_maps,
    (
         SELECT count(layer.id)
         FROM map INNER JOIN layer ON (layer.map_id = map.id)
         WHERE map.client_id = client.id
    ) AS n_layers
FROM client;